dev-qt/qt-creator's ClangFormat plugin, unfortunately, requires clang be patched to support an option "KeepLineBreaksForNonEmptyLines", used by the IDE to inhibit the removal of line breaks. Though it seems unlikely that clang will accept the patch upstream (see https://reviews.llvm.org/D53072) the qt-creator plugin is no longer marked as experimental and QT devs expect downstream to ship qt-creator with its own bundled LLVM/Clang, built from QT's own LLVM/Clang fork. Bundling violates Gentoo's philosophy. There is no aesthetically pleasing solution to this problem. Disabling the plugin at build-time or allowing the plugin to auto-disable itself at runtime will impact the end user's experience by failing to support an important and expected feature for modern IDEs. Given that the default behavior of libClangFormat and clang-format would remain the same when the option "KeepLineBreaksForNonEmptyLines" is not used, I see its inclusion in sys-devel/clang to be the lesser of all evils.
As you said, it's against Gentoo philosophy. I won't make Gentoo depend on a rejected fork of clang. Qt devs have to figure this out.
For other people looking at this: This was solved by adding the dev-libs/libclangformat-ide package that Qt Creator depends on. PS! The dev-libs/libclangformat-ide package has recently been removed - see https://bugs.gentoo.org/734732
